A tandem type color laser printer includes a plurality (four) of photosensitive drums and developing rollers for respective colors, i.e., yellow, magenta, cyan, and black.
In the tandem type color laser printer, a toner image of each color is formed on the corresponding photosensitive drum by the corresponding developing roller substantially simultaneously, and thereafter, each toner image is transferred on a sheet or an intermediate transfer belt. Accordingly, a color image can be formed at substantially same speed as that of a monochrome laser printer.
As such a tandem type color laser printer, for example, JP-A-2006-98772 describes that an image forming unit including a plurality of photosensitive drums and developing cartridges can be pulled from and removably mounted on a main body casing.